A known magnetic position sensing apparatus senses a position of a sensing subject, which moves relative to a reference member. This type of position sensing apparatus includes two magnets, two yokes (magnetic flux conducting members) and a magnetic flux density sensing device (magnetic flux sensing means). The magnets and the yokes form a closed magnetic circuit. The magnetic flux density sensing device is, for example, a Hall element that is installed in an inside of the closed magnetic circuit. For example, the magnets and the yokes are installed to the sensing subject, and the magnetic flux density sensing device is installed to the reference member. A density of the magnetic flux, which passes through the magnetic flux density sensing device, changes in response to a position of the magnetic flux density sensing device relative to the yokes. The position sensing apparatus senses the position of the sensing subject based on the density of the magnetic flux, which passes through the magnetic flux density sensing device.
For example, a position sensing apparatus disclosed in JP2009-085913A senses a rotational angle of a sensing subject, which rotates relative to a reference member. In the position sensing apparatus, two yokes extend in parallel with each other in a rotational direction of the sensing subject and are opposed to each other in a direction, which is referred to as an opposing direction. When the yokes are viewed in the opposing direction, a width of each yoke progressively decreases from each of two end portions of the yoke toward a center of the yoke. With this construction, a leakage magnetic flux, which is present in a gap between the yokes, is increased, and thereby it is possible to improve linearity of an output signal of the magnetic flux density sensing apparatus, i.e., it is possible to approximate a relationship between the rotational angle of the sensing subject and the output voltage of the magnetic flux density sensing device to a linear relationship.
The width of the center portion of the yoke, which is located in the center in the rotational direction, is relatively small in comparison to the width of the end portions of the yoke, which are opposite to each other in the rotational direction. Therefore, the position sensing apparatus of JP2009-085913A is vulnerable to the disturbance caused by an external magnetic field. Thereby, in order to avoid the influence of the external magnetic field, the width of the center portion of the yoke cannot be reduced from a predetermined minimum width, which is required to provide sufficient resistivity against the disturbance. Thus, there is a limit with respect to the increase of the amount of the leakage magnetic flux in the gap between the yokes. Furthermore, in a case where the width of the center portion of the yoke, which is designed to provide a good linearity of the output signal of the magnetic flux density sensing device, is smaller than the predetermined minimum width, which is required to provide the sufficient resistivity against the disturbance, the width of the center portion of the yoke has to be changed while sacrificing the linearity of the output signal.
Furthermore, it is difficult to design and manufacture the yoke that has the width, which progressively changes.